MILAN—Diseases rarely seen in modern society are popping back up in Italyand elsewhere in Europe, where a slew of rumors linking vaccines to supposed complications like autism are spreading with the help of populist politicians.

The most prominent backer of such rumors here is Beppe Grillo, Italy’s most popular politician and the leader of the populist Five Star Movement.

“Vaccines have played a fundamental role in eradicating terrible illnesses such as polio, diphtheria and hepatitis,” Grillo wrote on his blog in 2015, when he first seized on the issue. “However, they bring a risk associated with side effects that are usually temporary and surmountable… but in very rare cases, can be as severe as getting the same disease you’re trying to be immune to.”

Despite all the qualifiers, the message was one of fear and conspiracy, always useful to those who want to rally the masses. Grillo was riding a wave.

As The Daily Beast reported in 2015, that year was supposed to be the year that measles was eradicated in Europe. The goal was set a decade earlier when vaccines, though not mandatory, were not-so-subtly required for school admission in a growing number of European countries. But instead of eradication, Europe faced one of the worst outbreaks of the preventable diseases of measles and rubella in recent memory. The number of cases of measles in Europe grew by 348 percent after 2007, climbing from 7,073 cases then to 31,685 cases in 2013 according to the World Health Organization. For the moment, Europe is polio free, and globally the crippling, deadly disease has been on the way to eradication—thanks to vaccinations. But the measles example shows how quickly such gains can be reversed.

Grillo’s party picked up on the issue and the ensuing polarization over vaccines has had a palpable effect on Italian society, taking a bad situation and making it all the worse.

The Five Star Movement (or 5SM, if you will) is Italy’s most popular political movement. Populist, euro-skeptics who bill themselves as outsiders, the 5SM began in 2009 when Grillo, now 68, and his web-strategist colleague, Gianroberto Casaleggio, began hosting gatherings through the website Meetup.com to draw out disillusioned Italian youth.

Grillo’s fame derived from his stinging political satire as a TV comedian in the ’80s. He was banned from television in 1987 for his critiques of the establishment and waited in obscurity for almost 20 years. Then, in 2005, he roared back into public consciousness when he took out a full-page ad in the Italian daily La Repubblica calling for the resignation of then-Italian Central Bank Governor Antonio Fazio, and another full-page ad in the International Herald Tribune demanding a ban on parliamentarians with criminal records. These exploits led to Time magazine naming Grillo a “European Hero.”

Grillo’s personal star and the 5SM’s internet savviness fueled their meteoric rise over the next few years. Their ability to tap into the disenchantment caused by the political establishment also contributed to their success, as did their publicizing of environment issues that appeal directly to Italy’s youth.

In 2013 they were the most-voted-for party in Italy. And despite a sluggish performance in local elections last month, the 5SM is expected to be a major player in next year’s elections for parliament.

With all that power, Grillo often uses his influence to propagate fake news and disseminate obscure weblinks to conspiracy theories.

Grillo regularly posts questionably sourced stories on his blog, which is said to be one of the top 10 read websites on the planet. His anti-vaccination propaganda, in particular, drew condemnation in a New York Times editorial published this past May.

He is not alone, as we know. The ranks of anti-vaxxers in the United States have included Jim Carrey, Jenny McCarthy, and even American President Donald Trump.

Earlier this year, @realDonaldTrump wrote on his Twitter account, “Healthy young child goes to doctor, gets pumped with massive shot of many vaccines, doesn’t feel good and changes - AUTISM. Many such cases!”

Such issues are becoming more common across Europe. In France, more than 20 percent of the population does not support vaccinations. Marine Le Pen, the leader of far right party the National Front, said in early July she was “completely opposed” to mandatory vaccinations. Such attitudes have also caught on in recent years in, Poland, Romania, Switzerland, Ukraine, Austria, and Germany, where 504 cases of measles were reported by mid-April (compared to just 33 cases from the same period in 2016).

In Italy, the first half of 2017 saw the number of measles cases triple from 2016. Out of 3,000 new cases of measles recorded through June, 40 percent of the infected faced complications. Italy’s last outbreak of measles saw 18,000 registered cases and led to 15 deaths. The fallout for doubting vaccines has made such an impact that a few Five Star Movement members have walked back vaccine doubts. Paola Ferrara, a Five Star Movement member who works in the city hall in Rome, said in May she considered vaccinations “essential.”

But vaccines aside, the 5SM does plenty of damage to Italian society by propagating other baseless rumors. In May, other high-ranking members of the 5SM helped propagate a vicious conspiracy targeting NGOs and charities working with refugees. 5SM repeated claims by a public prosecutor from Sicily that accused groups of colluding with human traffickers with an aim of unsettling the Italian economy. No evidence has been found to support this claim.

While some members have walked back to the center on vaccines, the movement has shifted further right on a number of issues including on receiving migrants. Rome Mayor Virginia Raggi said her city could no longer afford the “devastating social cost” of receiving refugees. Only a few months earlier she had delivered a different message:

“We as mayors and our cities face the effects of large immigration inflows. It is our duty to guarantee dignity, shelter and human warmth to newcomers. Negative attitudes and closure offend our human dignity.”